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Hopefully a simple question

0

I'm hoping this is a simple question with a mundane answer!

I have 10 zebra danios, and have just noticed that one of them has gotten very skinny, just wondering as to possible reasons.

But also, another danio seems to be swimming strangely, it seems to be swimming almost vertically upwards before sort of drifting downwards, it seems that the fish is having to constantly work to stay up in the water.

Has anyone seen anything like this? And is there Anything to worry about?

Hi Pubber.
Tell us about your tank to help figure things out. how many gallons, how many fish and what kind, what does your Ammonia, nitrites and nitrates read?
Is the tank cycled and if so for how long? What kind of filtration do you have and what's your maintenance schedule like (as in how often do you do water changes and how much do you change out?) this will give us an idea of what might be happening.

Tank has been cycled for about 5 months, and fish only get added every 3-4 weeks!! In terms of params, I don't do my own tests as I'm a beginner, my Lfs do them for me. The tests always show good levels of all of them, but I can't give specifics!!

I carry out weekly 25% wc's, and have been carrying out more frequent ones recently due to a slight bacteria bloom (this cleared up in a week)

Hum. I would strongly suggest you get an API liquid test kit and start testing your water yourself. WE all test ourselves here because it's the only way to insure we know what our water perimeters actually are. I wasn't a beginner that long ago but have always tested myself.
The problem is, you can have ammonia spikes and not realize it. Depending on when you take the water to your LFS (before or after a water change) you could get readings that do not really reflect what's happening in your tank. Also, if you've recently had a bacterial bloom, it would suggest to me that perhaps your tank might not be fully cycled or you're experiencing a mini cycle.
It's critical to know what your ammonia, nitrites and nitrates read. Without knowing exactly what they are, if I were to hazard a guess, I'd say your danios might be suffering from ammonia or nitrate poisoning.
I'd start upping the water change to 50%. And I'd do one asap just in case you are dealing with an ammonia issue to try to get the level down.
Again - get that API liquid test kit. Generally the LFS uses test strips which are highly inaccurate so them telling you that your water is 'okay' really doesn't mean much.

Something else to remember - you're beneficial bacteria lives in your filters, not in your water so removing water cannot affect your cycle.

have you cleaned your filter since you set the tank up? And if so, how did you clean it? did you use tap water or old tank water?
is chlorine an issue where you live? if so, do you use a dechlorinator each time you do a water change?

++ 1 to all of the above to fishmommie. For alot of years all I tested was the pH in my tanks and learned the hard way when an ammonia spike wiped out a whole tank last year. Know I test everything weekly.

Perfection is not attainable, but if we chase perfection we can catch excellence.
Once you learn to quit, it becomes a habit. -Vince Lombardi
“Be more concerned with your character than your reputation, because your character is what you really are, while your reputation is merely what others think you are.” ― John WoodenSandy Hook Elementary......Lest We Forget
See my profile for my tanks and what fish I keep

it does sound to me like both of your danios are dying. This may be due to issues that are beyond your control but it would behoove you to take the advice the folks here are giving to, so you can find out if the problem is something you could be preventing.